Magnetic Force Microscopy (hereinafter referred to as MFM) and Magnetic Resonance Force Microscopy (hereinafter referred to as MRFM) provide micrometer-scale imaging of magnetic structures and surfaces. In MFM, a ferromagnet-tipped cantilever is brought into close proximity with a sample surface to detect the force between the tip and the sample. The tip is scanned over the surface to reveal the magnetic domain structure of the sample. A typical application of MFM is in data storage technology, such as magnetic disk drives. MRFM is potentially significantly more sensitive than MFM, with capability of providing nanometer-scale three-dimensional (3D) imaging of small structures such as semiconductor quantum dots (e.g., for quantum computing) and biological samples such as cells, proteins and DNA.
MRFM is a combination of Nuclear Magnetic Resonance Imaging (MRI) and Atomic Force Microscopy (AFM). A Magnetic Resonance Force Microscope (MRFM) is a microscopic imaging instrument that mechanically detects magnetic resonance signals by measuring the force between a permanent magnet and spin magnetization. Conventional MRI is able to provide images of muscular tissue, for example, by measuring changes to a voltage induced in a coil inductor when the magnetic spins of the atoms in the tissue are excited by a radio frequency (RF) magnetic field. The RF field is driven at the natural or “resonance” frequency of the spins, causing them to rotate or precess about a strong static magnetic field. The spins in the case of human MRI studies are those of the hydrogen nuclei (protons) in the fat and water in the body (the human body is about ⅔ hydrogen). The imaging occurs when a gradient, or spatially varying static field is used, such that only a small slice of the specimen is in resonance with the RF field at any given time. The position of this slice is often controllably varied, yielding a position-sensitive measurement of the resonant spin domain (an MRI image). Thus, MRI is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. The spatial resolution of MRI is about 0.1 millimeter (mm) or perhaps slightly less (10 μm resolution has been achieved in a lab based non-commercial NMR microscope).
AFM is fashioned after the scanning tunneling microscope (STM). AFM has the capability of imaging individual atoms on the surface of a material by measuring the atomic-scale repulsive force between the atoms themselves and the tip of a compliant cantilever, usually made of silicon or silicon-nitride. When brought extremely close to the surface under study (of order 1 nanometer), the interaction forces between the surface and tip cause the cantilever to deflect or bend. This deflection is then measured, usually by reflecting a laser beam off the back of the cantilever and toward a photodiode detector. The AFM can accurately image structures down to the Angstrom scale (10−10 m), about a million times smaller than that of MRI.
Both an MFM and an MRFM device typically comprise a small ferromagnet that is attached to the terminal end of an AFM cantilever. This ferromagnet generates an inhomogeneous magnetic field (a gradient field), whereby the magnetic field of the ferromagnet decreases sharply with increasing distance from the cantilever. When a magnetic moment M is exposed to a gradient magnetic field (δB/δr), it experiences a force F equal to the product of the moment and the gradient (F=M δB/δr). If the AFM cantilever with the associated magnetic tip is positioned near the surface of a specimen material containing a plurality of magnetic moments (spins), the possibility exists for those spins to feel the magnetic gradient δB/δr and thereby the force F. This in turn causes the cantilever to feel an equal and oppositely directed force, causing it to deflect. Thus, the cantilever senses the presence of magnetic spins at and, in the case of MRFM, even beneath the specimen surface.
The relative positions of the cantilever and the specimen may be changed, or scanned, in an MFM or MRFM device, to yield a spatial map of the force F experienced by the cantilever, which translates as a spatial map of the underlying magnetic spin structure of the specimen. In addition to lateral and vertical scanning typical of an AFM device, which provides a topographic map of the surface of a specimen, an MRFM device provides additional provides vertical scan information, resulting in three-dimensional imaging of the specimen with sub-surface capability similar to MRI, but with AFM-scale resolution.
The ultimate spatial and magnetic moment resolutions of both the MFM device and the MRFM device are determined by the magnitude of the magnetic field gradient δB/δr, the mechanical limitations of the cantilever, and the sensitivity of the cantilever motion detector. Smaller physical dimensions of the cantilever are highly desirable to enable imaging of smaller particles such as cells and proteins and DNA. However, the present state of the art detection scheme employs laser light directed at and reflected off the backside of the cantilever, toward a photodetector or interferometer. As the cantilever size decreases, optical detection becomes increasingly difficult, especially when the cantilever dimensions approach or become less than the wavelength of the light in the detector beam. The use of micro-scale cantilevers is a major factor in limiting MFM and especially MRFM resolution in present devices, which is presently at the 10,000 to 100,000 spin level. Micro-dimensional probes that are capable of detecting single proton and single electron spin are therefore, not possible using present cantilevers.